Cumulonimbus cloud

According to the definition by the World Meteorological Organization:
Heavy and dense cloud, with a considerable vertical extent, in the form of a mountain or huge towers. At least part of its upper portion is usually smooth, or fibrous or striated, and nearly always flattened; this part often spreads out in the shape of an anvil or vast plume.  Under the base of this cloud, which is often very dark, there are frequently low, ragged clouds, either merged with it or not, and precipitation in the form of virga.
These clouds are capable of producing lightning and other dangerous severe weather, such as storm, tornadoes and hailstones.
This cloud may look beautiful, but it is very scary and dangerous in aviation, and pilots avoid entering it and even approaching it. Because severe turbulence inside it can be very dangerous.